The present invention relates to communications systems, and more particularly to data over voice (DOV) communications equipment which simultaneously transmits and receives voice and data signals through a common 2-wire signal line.
The DOV communications system is a known technique for simultaneously transmitting and receiving an analog voice signal and a digital data signal (e.g., a control signal, other general data information, etc.) through a common 2-wire signal line. The DOV communications system is a so-called band division system, in which the digital data is FSK (Frequency Shift Keying) modulated on a high frequency carrier signal and is superposed on a signal line. Therefore, it is easy to separate the high frequency FSK modulated data signal from the low frequency voice signal by a filter cirucit.
FIG. 1 is a block diagram of an exchange (EX) side of a prior art DOV system. Voice signals and FSK modulated data signals are received from a terminal (TE) side of the system through a plurality of 2-wire signal lines L.sub.1, L.sub.2, L.sub.3, . . . L.sub.N. The signals are received as a voltage difference between the signal lines 8 and 9 and are converted to 4-wire signals by a hybrid circuit 38. From the exchange (EX) side, a DC (Direct Current) voltage -V is provided to the terminal through the 2-wire signal lines L.sub.1, L.sub.2, L.sub.3, . . . , L.sub.N. A capacitor 39 is connected to the teminal side of the hybrid circuit 38 to allow transmission of the voice signals and data signals to the 4-wire signals lines. On the exchange side of the hybrid circuit 38, a center tap of a hybrid coil 41 is coupled to a balancing resistor. If the balancing resistor is theoretically modulated in order to match the impedance observed from the side of the 4-wire signal line to the impedance observed from the side of the 2-wire signal line, then the signals transmitted from the terminal side only appear at a pair of signal lines S1-R (S2-R, S3-R, . . . , SN-R) of the 4-wire signal lines 50-1 (50-2, 50-3, . . . , 50-N). Similarly, signals which are transmitted from the exchange side through a pair of signal lines S1-T (S2-T, S3-T, . . . , SN-T) of the 4-wire signal lines 50-1 (50-2, 50-3, . . . , 50-N) do not appear at the pair of signal lines S1-T (S2-T, S3-T . . . , SN-T).
Therefore, a voice signal which is transmitted on the pair of signal lines S1-T (for exemplary purposes, only the L1 set of lines is considered here and below) is filtered by a filter 10 and converted to a digital signal by an A/D converter 12. The digital signal is then multiplexed by a multiplexer 14, sent on a highway THW and processed by other portions of the exchange (not shown). A voice signal which is sent from other portions of the exchange side through a highway RHW is distributed to an appropriate line by a demultiplexer 16. When it is determined that the digital signal from the highway RHW should be distributed to the 2-wire signal line L1, the digital signal is sent to a D/A converter 18 (where it is converted to an analog signal), and the analog signal is sent to the 2-wire signal line L1 through a filter 20 and the hybrid circuit (i.e., 4-wire/2-wire converter) 38.
On the other hand, a data signal which is transmitted on the 2-wire signal line L1, is provided to a pair of signal lines S1-R of the set of 4-wire signal lines 50-1, via the hybrid circuit 38, in the same way as the voice signal. The data signal consists of a FSK modulated signal which is obtained by modulating the carrier with data at a higher frequency than the voice frequency band. The data signal is rejected by the filter 10 which is tuned for the voice signal, but is sent to a demodulator 24 through a filter 22. The demodulated output is passed through a selector 80 into a microcomputer 56. Thereafter, the signal enters a FIFO (First In First Out) register 26 and is sent to a control unit (not shown) of the exchange through a scanning multiplexer 28. A data signal which is sent from the control unit is input to a shift register 81 and is then sent to the 2-wire signal line L1 through a FIFO register 30, the selector 80, the microcomputer 56, an AND gate 32, a modulator 34, a filter 36 and the hybrid circuit 38. The signal flow is identical for the other 2-wire signal lines.
FIG. 2 is a block diagram of the terminal (TE) side and the exchange (EX) side of a prior art DOV system. At a hybrid circuit 38A of the terminal side, a DC power supply cirucit (i.e., DC/DC converter) 83 is connected to a hybrid coil 84. The DC power supply circuit 83 is supplied with DC power from the power source -V of the exchange side through the 2-wire signal line L1, and supplies DC power to a control circuit C2 and to other circuits (not shown) which require DC power. Filters 10A and 20A are low pass filters and filters 22A and 36A are bandpass filters. These filters are provided for both the voice signal and the data signal systems and in both the transmitting and receiving directions. TD represents data to be transmitted, while RD represents received data. The exchange side illustrated in FIG. 2 is as described with respect to FIG. 1.
In the prior art equipment, as described above, the hybrid circuit 38 (38A) is required to convert both data and voice signals from 2-wire signals to 4-wire signals and vice versa. Therefore, the bandwidth of the hybrid circuit 38 (38A) is required to be wide, so that the hybrid circuit 38 (38A) is large and expensive. Moreover, the filters 10 and 20 (10A and 20A) are required for transmitting and receiving a voice signal, while filters 22 and 36 (22A and 36A) are required for transmitting and receiving a data signal. Further, these filters are required for each subscriber line in addition to the demodulator 24 and modulator 34. Accordingly, for N subscriber lines, N sets of hardware are required, thereby increasing the size of the equipment, the consumption of electrical power and the cost. Therefore, there has been a need in the art for a simplified and less expensive hybrid circuit for use in DOV communications systems.